1. Field of the Invention:
The present invention relates to the field of fluid shear couplings, and particularly to a novel design for a free floating dam used to provide return of shear fluid from the working chamber.
2. Description of the Prior Art:
Fluid shear couplings, often referred to as "viscous fan drives", are used in a variety of applications. The fan drive includes a working chamber defined by the cavity created by the bearing housing and cover assembly. The silicone fluid in this chamber will transmit torque from a rotor to the bearing housing which is physically attached to a fan blade assembly. The amount of silicone fluid in the working chamber dictates the fan speed due to fluid shearing action between the rotor and the walls of the working chamber. The difference between the engaged and disengaged modes for a fan drive is directly proportional to the amount of torque transmitting silicone fluid present in the working chamber.
Fluid shear couplings are typically used to drive engine cooling fans on automotive vehicles. These couplings have a disengaged mode in which the fan is rotated at a relatively low angular velocity and an engaged mode in which the angular velocity is relatively high. For fuel conservation and noise reduction, it is preferred to have the disengaged mode at as low an angular velocity as possible. Under certain conditions, it is desirable to have the fluid pumped out of the working chamber and into the reservoir or to a recirculation passageway as quickly as possible.
Dam systems used to pump fluid out of the working chamber are normally placed near the outside diameter of the rotor. The purpose of the dam is to build fluid pressure at the reservoir return passageway, commonly referred to as the "dump hole". This fluid pressure is the result of the differential speed between the rotor (at engine speed) and the cover (at fan speed), the viscosity of the silicone fluid, and the clearance between the rotor and the dam. The fluid pressure at the dam will cause the fluid to flow back to the reservoir through the passageway. The amount of fluid pressure available due to the indicated parameters is an indication of the efficiency of the dam. The level of fluid in the working chamber is due to the dam's efficiency in a steady-state and results in the disengaged fan speed.
Two types of dam designs known in the prior art are fixed clearance dams and adjustable spring-loaded dams. A third design is the free floating dam which allows greater tolerances to be permitted without the complexity of springs. For fixed clearance dams, there is a requirement for tight control of the rotor, cover and dam assembly tolerances to maintain a close, fixed dam clearance. Such dams require attachment to the cover, by roll pins, etc. Spring-loaded dams permit looser control of the rotor and cover tolerances since the dam adjusts to compensate for the resulting clearances. However, the design requires extra parts such as springs for the adjustment, and assembly can be difficult and tedious. Free floating dams allow for looser control of the rotor and cover tolerances without the requirement for springs. Assembly is also easier.
In U.S. Pat. No. 4,271,946, issued to Bridge on June 9, 1981, there is described a pumping or dam element for a temperature responsive viscous fan drive. The bridge device is typical including a disc which rotates within a housing that defines a return hole adjacent the outer portion of the disc. An L-shaped pumping element is described which includes a first portion along the circumferential wall at the outer perimeter of the disc and a second portion in an annular groove at the side of the disc adjacent to the return hole. The L-shaped dam is welded to the valve plate through which the return hole extends.
A fluid shear coupling having two dam elements oriented perpendicular to one another is disclosed in U.S. Pat. No. 4,383,597, issued to Blair on May 17, 1983. The Blair apparatus includes a rotor received within a working chamber and a return hole extending in through one side of the housing next to the outer portion of the disc. One of the dam elements comprises a projection which is fixed in the cylindrical cavity adjacent the outer periphery of the disc, and includes an angled surface to direct fluid to the side of the housing where the return hole is located. A separate dam is attached in an annular groove adjacent the outer portion of the disc and includes a surface surrounding the return hole to direct fluid into the hole in conventional fashion. In the Blair device, the two dam elements are separate and each is attached to the housing.
A variety of other dam designs are also known in the art. For example, in U.S. Pat. No. 4,564,094, issued to Storz on Jan. 14, 1986, there are shown various shaped planar dam structures with tabs received in a slot, which allows the dam to move circumferentially between two positions. The two positions correspond to placement of the dam in relation to one of two different return dump holes, depending on the relative rotation of the rotor and housing. Other examples of dam designs are shown in U.S. Pat. Nos. 4,564,093 and 4,485,902, issued to Storz on Jan. 14, 1986 and Dec. 4, 1984, respectively; and 4,086,988, issued to Spence on May 2, 1978.